This invention relates generally to image-forming machines with densitometers. More particularly, this invention relates to electrophotographic image-forming machines with on-line densitometers.
Electrophotographic (EP) image-forming machines are used to transfer images onto paper or other medium. An EP image-forming machine usually has a photoconductor with a film, which is electrostatically charged and optically exposed to form an electrostatic latent image on the surface. Toner is deposited onto the photoconductor. The toner is charged, thus adhering to the photoconductor surface in areas corresponding to the electrostatic latent image. The toner image is transferred to the paper or other medium. The paper is heated for the toner to fuse to the paper. The photoconductor is then refreshedxe2x80x94cleaned to remove any residual toner and chargexe2x80x94making it ready for another image.
Many EP image-forming machines have a densitometer to assist with operating and controlling the image-forming process. The densitometer determines the density of the toner on the photoconductor, from which operating adjustments are made. The densitometer typically is a transmission densitometer, which has an emitter and a collector on opposite sides of the photoconductor. The emitter may be made of a GaAIAs chip having a wavelength of about 880 nm. The optical path passing between the emitter and collector also passes through the photoconductor and any toner on it. The densitometer provides a voltage reading corresponding to the amount of light energy passing from the emitter to the collector. The voltage reading also corresponds to the density of the photoconductor and any toner on it.
The densitometer usually works in conjunction with a process patch, which is on the surface of the photoconductor in an interframe or edge area. As the EP image-forming machine operates, the process patch is charged, exposed, and developed to provide the maximum toner density on the process patch. The densitometer provides the density at the process patch and at a place on the photoconductor without tonerxe2x80x94adjacent to the process patch and outside the image frames. The voltage reading of the photoconductor without toner is subtracted from the voltage reading of the process patch (photoconductor and toner) to provide the density of toner on the photoconductor process patch.
Photoconductors typically have three layersxe2x80x94a support underlayer made of polyester or similar material, a center conductive layer, and an electrographic surface or film. Older film designs are optically and electrostatically sensitive, but are essentially not sensitive to the infrared spectral region. The emitter and collector typically operate in the infrared range; thus making these older films relatively impervious to the emissions of the densitometer. While these films may experience some effect from the densitometer, these effects are relatively unnoticeable in the output image.
Recent film designs are sensitive to densitometers in current use. Newer films are sensitive to the infrared spectral region. Since densitometers are responsive to infrared emissions, the emitters may cause artifactsxe2x80x94unwanted imagesxe2x80x94to appear on the newer films and, consequently, the output images from the machine. The types of artifacts vary. However, densitometers generally make two types of artifacts. The first artifact appears as a small dark spot on the output image. When an image-forming machine stops, the densitometer exposes (damages) the film in a small spot. The second artifact appears as a dark line in the output image. With continuous running, the densitometer exposes (damages) the film along a continuous line. Artifacts are not acceptable for good image quality. These artifacts usually are not permanent, taking several xe2x80x9crefreshxe2x80x9d cycles to clean. To avoid artifacts, emitter and collector diodes may be chosen having a longer wavelength so as not to expose the film. However, these diodes may cost significantly more than diodes currently used.
This invention provides an image-forming machine with a pulse densitometer that avoids the formation of artifacts on electrophotographic films. The pulse densitometer may be used to provide pulsed emissions essentially below the exposure threshold of electrophotographic films, especially those sensitive to the infrared spectral region.
In one aspect, the image-forming machine includes a photoconductor having a film with an exposure threshold. One or more chargers, an exposure machine, a toning station, and a density measurement device are positioned adjacent to the photoconductor. The one or more chargers electrostatically charge the film. The exposure machine optically exposes and forms an electrostatic image on the film. The toning station applies toner onto the film. The toner has a charge to adhere to the electrostatic image. The density measurement device provides one or more pulses to measure the toner density, the photoconductor density, or a combination of the toner and photoconductor densities. The pulses are essentially less than the exposure threshold of the film.
In another aspect, the image-forming machine includes a photoconductor having a film with an exposure threshold. One or more chargers, an exposure machine, a toning station, and a densitometer are positioned adjacent to the photoconductor. The one or more chargers electrostatically charge the film. The exposure machine optically exposes and forms an electrostatic image on the film. The toning station applies toner onto the film. The toner has a charge to adhere to the electrostatic image. The densitometer has an emitter, a collector, and a pulse apparatus. The emitter and collector are positioned oppositely next to the photoconductor. The emitter provides one or more pulses in response to a drive signal. The one or more pulses are essentially less than the exposure threshold of the film. The one or more pulses measure the toner density, the photoconductor density, or a combination of the toner and photoconductor densities. The pulse apparatus is connected to provide the drive signal to the emitter according to a duty cycle.
In yet another aspect, an on-line densitometer for an image-forming machine has an emitter, a collector, and a pulse apparatus. The emitter provides one or more pulses according to a drive signal. The collector is positioned to receive the one or more pulses along an optical path with the emitter. The pulse apparatus connected to provide the drive signal to the emitter according to a duty cycle.
Other systems, methods, features, and advantages of the invention will be or will become apparent to one skilled in the art upon examination of the following figures and detailed description. All such additional systems, methods, features, and advantages are intended to be included within this description, within the scope of the invention, and protected by the accompanying claims.